1. Field of the Invention
The invention relates to an over-current protection apparatus, especially to an over-current protection apparatus for high voltage.
2. Description of Related Art
FIG. 1 is a conventional over-current protection apparatus 10, including an upper electrode foil 11, a lower electrode foil 12 and a current-sensing element 13. The upper electrode foil 11 and lower electrode foil 12 are metal conductive plates, respectively, and the current-sensing element 13 is formed by a conductive material with its resistance exhibiting a positive temperature coefficient (PTC) property. The conductive material with positive temperature coefficient refers to such material of which the resistance value is maintained at extremely low value under low temperature. However, when an over-current phenomenon occurs whereby the temperature is increased to a critical temperature, its resistance value will increase instantly thousands of times to a high resistance state, which can compensate for the over-current reversely and achieve the object of protecting circuit devices.
The current that the over-current protection apparatus can endure may be calculated by a general formula: V=IR. Therefore, to effectively protect the circuit devices and to endure a higher instant current, the requirement of high-voltage endurability for over-current protection apparatus becomes higher, particularly for the purpose of protecting the electronic communication product from a short circuit caused by an instant enormous amount of current produced by a lightning strike, which may even lead to an explosion.
Generally, the current-sensing element 13 of the over-current protection apparatus 10 may be formed by ceramic or conductive polymer materials. Although the ceramic current-sensing element has the characteristic of being able to endure high voltage (>600V) and may recover to its initial state. However, under a high or a low temperature condition, the resistance of the ceramic current-sensing element will appear a negative temperature coefficient phenomenon, and the resistance value of the element will reduce while the temperature rises, shown as curve A in FIG. 2. As a result, the current of the ceramic current-sensing element will increase with the rise of the temperature due to the negative temperature coefficient phenomenon, and that will result in a thermal run away phenomenon. When the temperature becomes out of control, the over-current protection apparatus may explode. Furthermore, since the ceramic current-sensing element is of lesser temperature sensitivity, it results in a longer time to trip. In addition, since the size of the over-current protection apparatus formed by ceramic material is so large that it is not suitable for the tendency of shrinking the size of electronic communication devices.
On the other hand, since the resistance value of the polymer current-sensing element formed by conductive polymer material does not have a negative temperature coefficient phenomenon and has a high switching off speed, it becomes the subject of intensive research and development at the present time. A diagram of the relationship between the resistance value thereof and the temperature is shown as curve B in FIG. 2. However, a normal polymer current-sensing element cannot endure high voltage (around 60V-250V). If the polymer current-sensing element needs to endure high voltage (>600V), then a lot of complicated processes are required. Furthermore, the polymer current-sensing element will lose its initial voltage after switching off and cannot restore to its initial state. Therefore, the polymer current-sensing element is not suitable for high voltage products. Table 1 shows a comparison on advantages and disadvantages between a ceramic current-sensing element and a polymer current-sensing element.
TABLE 1Ceramic current-sensingPolymer current-sensingelementelementAdvantage(a) it endures a high(a) it will not produce negativevoltage(>600 V);temperature coefficientphenomenon;(b) it can recover to an(b) it has a higher switching offinitial state after switchingspeed, and with a higheroff;temperature sensitivity;(c) the volume of the formedover-current protectionapparatus is smaller.Dis-(a) under a high or low(a) it will not endure highadvantagetemperature, the negativevoltage (its voltage endurabilitytemperature coefficientis generally around 60 V-250 V);phenomenon will occur;(b) it has a lower(b) after switching off, it willtemperature sensitivity,lose the intial voltage.resulting a slower switchingoff speed;(c) the volume of theformed over-currentprotection apparatus islarger
To sum up, it is necessary to provide a solution addressing the advantage and disadvantage of the ceramic current-sensing element and the polymer current-sensing element, so as to produce an over-current protection apparatus which has a high voltage endurability and can avoid thermal run away phenomenon.